Polyimide resin is widely used for electric and electronic purposes due to its excellence in heat resistance, electrical insulation reliability, chemical resistance, and mechanical properties. For example, polyimide resin is used as an insulating film and/or a protective coating agent on a semiconductor device, a substrate material and/or a surface protecting material for a flexible circuit board, an integrated circuit board, or the like. Alternatively, polyimide resin is used also to provide a microcircuit with an interlayer insulating film and/or a protective film.
In particular, in a case where polyimide resin is used as a surface protecting material for a flexible circuit board, a cover lay film obtainable by applying an adhesive to a molded article such as a polyimide film is used. The cover lay film is generally adhered to the flexible circuit board by the following method. That is, an opening is secured in advance, by punching or the like method, at a bonding portion between the cover lay film and a terminal portion or a component of a circuit, the opening and the cover lay film are aligned with each other, and the cover lay film and the flexible circuit board are then subjected to thermocompression bonding by a thermal press or the like.
It is, however, difficult to secure an opening in a thin cover lay film with high accuracy, and the alignment to bond the cover lay film and the flexible circuit board to each other is usually carried out manually. This causes insufficiently accurate alignment and low workability at the lamination, thus leading to increases in cost.
Meanwhile, a surface protecting material for a circuit board can be provided by using a method in which a resin composition, called a solder resist, having an insulation function is directly applied on a circuit board, and the resin composition is then cured to provide an insulating film. The solder resist has excellent flexibility and electrical insulation reliability as an insulating material. However, the solder resist has excess viscosity (poor tack property) caused after a coating film formed by applying the resin composition to a circuit board is dried, decreases in workability and yield, and/or process pollution.
There have been proposed various solder resists that have improved tack property while maintaining flexibility and electrical insulation reliability.
Further, there has been proposed a thermosetting resin composition that offers a good balance of properties such as printability, tack property, matting property, electrical insulation property, and adhesiveness to a coated product (see, for example, Patent Literature 1).